U.S. patent application number 13/624607 was filed with the patent office on 2013-04-11 for gas field ionization ion source, scanning charged particle microscope, optical axis adjustment method and specimen observation method.
This patent application is currently assigned to HITACHI HIGH-TECHNOLOGIES CORPORATION. The applicant listed for this patent is Hitachi High-Technologies Corporation. Invention is credited to Muneyuki FUKUDA, Tomihiro HASHIZUME, Tohru ISHITANI, Shinichi MATSUBARA, Yoichi OSE, Hiroyasu SHICHI.
Application Number | 20130087704 13/624607 |
Document ID | / |
Family ID | 40690218 |
Filed Date | 2013-04-11 |
United States Patent
Application |
20130087704 |
Kind Code |
A1 |
ISHITANI; Tohru ; et
al. |
April 11, 2013 |
GAS FIELD IONIZATION ION SOURCE, SCANNING CHARGED PARTICLE
MICROSCOPE, OPTICAL AXIS ADJUSTMENT METHOD AND SPECIMEN OBSERVATION
METHOD
Abstract
A gas field ionization ion source (GFIS) is characterized in
that the aperture diameter of the extraction electrode can be set
to any of at least two different values or the distance from the
apex of the emitter to the extraction electrode can be set to any
of at least two different values. In addition, solid nitrogen is
used for cooling. It may be possible to not only let divergently
emitted ions go through the aperture of the extraction electrode
but also, in behalf of differential pumping, reduce the diameter of
the aperture. In addition, it may be possible to reduce the
physical vibration of the cooling means. Consequently, it may be
possible to provide a highly stable GFIS and a scanning charged
particle microscope equipped with such a GFIS.
Inventors: |
ISHITANI; Tohru;
(Hitachinaka, JP) ; OSE; Yoichi; (Mito, JP)
; SHICHI; Hiroyasu; (Tokyo, JP) ; MATSUBARA;
Shinichi; (Komae, JP) ; HASHIZUME; Tomihiro;
(Hiki-gun, JP) ; FUKUDA; Muneyuki; (Kokubunji,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hitachi High-Technologies Corporation; |
Tokyo |
|
JP |
|
|
Assignee: |
HITACHI HIGH-TECHNOLOGIES
CORPORATION
Tokyo
JP
|
Family ID: |
40690218 |
Appl. No.: |
13/624607 |
Filed: |
September 21, 2012 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12314553 |
Dec 12, 2008 |
|
|
|
13624607 |
|
|
|
|
Current U.S.
Class: |
250/307 ;
250/306; 250/423F |
Current CPC
Class: |
H01J 2237/032 20130101;
H01J 37/08 20130101; H01J 2237/002 20130101; H01J 2237/0458
20130101; H01J 2237/061 20130101; H01J 37/18 20130101; H01J 37/28
20130101; H01J 2237/0807 20130101; H01J 37/285 20130101; H01J
2237/1501 20130101; H01J 27/024 20130101; H01J 2237/24514 20130101;
H01J 2237/0216 20130101; H01J 27/26 20130101; B82Y 15/00 20130101;
H01J 2237/024 20130101; H01J 2237/188 20130101; H01J 37/09
20130101; H01J 2237/0835 20130101 |
Class at
Publication: |
250/307 ;
250/423.F; 250/306 |
International
Class: |
H01J 37/285 20060101
H01J037/285; H01J 27/26 20060101 H01J027/26 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 14, 2007 |
JP |
2007-322703 |
Claims
1. A gas field ionization ion source, comprising: a needle-shaped
anode emitter; and an extraction electrode which forms an electric
field by which gas molecules at the apex of the emitter are ionized
and extracted; wherein the diameter of the extraction electrode's
aperture for letting extracted ions pass therethrough can be set to
any of at least two different values.
2. A gas field ionization ion source, comprising: a needle-shaped
anode emitter; and an extraction electrode which forms an electric
field by which gas molecules at the apex of the emitter are ionized
and extracted; wherein the extraction electrode can be separated
into an aperture-forming part having an aperture for letting
extracted ions pass therethrough, and a base part on which the
aperture-forming part is mounted, and wherein the aperture-forming
part can be withdrawn from and set around the optical axis of
ions.
3. A gas field ionization source according to claim 2 wherein, the
aperture-forming part is slid with respect to the base part.
4. A gas field ionization ion source, comprising: a needle-shaped
anode emitter; and an extraction electrode which forms an electric
field by which gas molecules at the apex of the emitter are ionized
and extracted; wherein the distance from the apex of the emitter to
the extraction electrode can be set to any of at least two
different values.
5-6. (canceled)
7. A scanning charged particle microscope, comprising: a gas field
ionization ion source having a needle-shaped anode emitter, and an
extraction electrode which forms an electric field by which gas
molecules at the apex of the emitter are ionized and extracted,
wherein the diameter of the extraction electrode's aperture for
letting extracted ions pass therethrough can be set to any of at
least two different values; a lens system by which ions from the
ion source are accelerated and focused on a specimen; a limiting
apparatus plate for limiting the ions which are focused on the
specimen; and a charged particle detector to detect charged
particles emitted from the specimen.
8. A scanning charged particle microscope, comprising: a gas field
ionization ion source having a needle-shaped anode emitter, and an
extraction electrode which forms an electric field by which gas
molecules at the apex of the emitter are ionized and extracted,
wherein the extraction electrode can be separated into an
aperture-forming part having an aperture for letting extracted ions
pass therethrough, and a base part on which the aperture-forming
part is mounted, wherein the aperture-forming part can be withdrawn
from and set around the optical axis of ions; a lens system by
which ions from the ion source are accelerated and focused on a
specimen; a limiting apparatus plate for limiting the ions which
are focused on the specimen; and a charged particle detector to
detect charged particles emitted from the specimen.
9. A scanning charged particle microscope according to claim 8
wherein, the aperture-forming part is slid with respect to the base
part.
10. A scanning charged particle microscope, comprising: a gas field
ionization ion source having a needle-shaped anode emitter, and an
extraction electrode which forms an electric field by which gas
molecules at the apex of the emitter are ionized and extracted,
wherein the distance from the apex of the emitter to the extraction
electrode can be set to any of at least two different values; a
lens system by which ions from the ion source are accelerated and
focused on a specimen; a limiting apparatus plate for limiting the
ions which are focused on the specimen; and a charged particle
detector to detect charged particles emitted from the specimen.
11-12. (canceled)
13. A method for adjusting the optical axis of a scanning charged
particle microscope comprising: a gas field ionization ion source
having a needle-shaped anode emitter, and an extraction electrode
which forms an electric field by which gas molecules at the apex of
the emitter are ionized and extracted; a lens system by which ions
from the ion source are accelerated and focused on a specimen; a
limiting apparatus plate for limiting the ions which are focused on
the specimen; and a charged particle detector to detect charged
particles emitted from the specimen; wherein the angular range of
emitted ions allowed to pass through the extraction electrode is
set larger for adjusting the optical axis of the gas field
ionization ion source but smaller than for adjusting the optical
axis for using the scanning charged particle microscope to observe
the specimen.
14. A method for observing a specimen by using a scanning charged
particle microscope comprising: a gas field ionization ion source
having a needle-shaped anode emitter, and an extraction electrode
which forms an electric field by which gas molecules at the apex of
the emitter are ionized and extracted; a lens system by which ions
from the ion source are accelerated and focused on a specimen; a
limiting apparatus plate for limiting the ions which are focused on
the specimen; and a charged particle detector to detect charged
particles emitted from the specimen; wherein the angular range of
emitted ions allowed to pass through the extraction electrode is
set larger for adjusting the optical axis of the gas field
ionization ion source but smaller than for adjusting the optical
axis for using the scanning charged particle microscope to observe
the specimen.
Description
RELATED APPLICATIONS
[0001] This application is the U.S. National Phase under 35 U.S.C.
.sctn.371 of International Application No. PCT/JP2011/064478, filed
on Jun. 23, 2011, which in turn claims the benefit of Japanese
Application No. 2010-151119, filed on Jul. 1, 2010, and Japanese
Application No. 2010-238711, filed on Oct. 25, 2010, the
disclosures of which Applications are incorporated by reference
herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to charged particle
microscopes for observing surfaces of such specimens as
semiconductor devices and new materials. For example, the invention
relates to a scanning charged particle microscope which uses light
ions as charged particles for shallow surface observation of a
specimen with a high resolution and a large depth of focus, and a
gas field ionization ion source for generating ions therein.
[0004] 2. Description of the Related Art
[0005] Non-Patent Document 1 describes a focused ion beam
(abbreviated to FIB) apparatus which is equipped with a gas field
ionization ion source (abbreviated to GFIS) and which uses hydrogen
(H.sub.2), helium (He), neon (Ne) or other gas ions. Unlike a
gallium (Ga: metal) FIB formed from the liquid metal ion source
(abbreviated to LMIS) which is used often these days, such a gas
FIB does not contaminate the specimen with Ga. It is also described
that since the energy spread of the gas ions extracted from a GFIS
is narrow and the virtual source size of the GFIS is small, it is
possible to form a smaller beam than the Ga-FIB. Especially, it is
further disclosed that the GFIS can attain better ion source
characteristics such as a higher angular current density if a fine
projection (hereinafter denoted as nano tip) is formed at the apex
of the emitter (or the atoms at the apex of the emitter are reduced
to several or fewer atoms). The phenomenon that a nano tip at the
apex of the ion emitter raises the angular ion current density is
also disclosed in Non-Patent Documents 2 and 3 and Patent Document
1. Examples of fabricating such nano tips are disclosed in Patent
Document 2 and Non-Patent Documents 3 and 4. In Patent Document 2,
a nano tip is formed by field evaporation from the emitter material
tungsten (W). In Non-Patent Documents 3 and 4, a nano tip is formed
of a second material which is different from a first metal or the
emitter material.
[0006] Each of Non-Patent Document 2 and Patent Document 2
discloses a scanning charged particle microscope provided with a
GFIS which emits ions of the light element He. Considering
irradiation particles in weight, a He ion is about 7000 times
heavier than an electron and about 17 times lighter than a Ga ion.
Therefore, the damage given to the specimen by a He ion, which is
dependent upon the magnitude of the momentum transferred to atoms
of the specimen, is a little larger than by an electron but greatly
smaller than by a Ga ion. In addition, the secondary electron
excitation regions resulting from irradiation particles penetrating
into the specimen are more localized to the specimen surface as
compared with those resulting from irradiation electrons. Due to
this characteristic, imaging by the scanning ion microscope
(abbreviated to SIM) is expected to be more highly sensitive to the
specimen's surface information than the scanning electron
microscope (abbreviated to SEM). Further from the viewpoint of
microscopy, ion beam irradiation is characterized in that imaging
can be done with a very large depth of focus since ions are so
heavier than electrons that the diffraction effects during focusing
of the ion beam is ignorable.
[Patent Document 1)
[0007] JP-A-1983-85242
[Patent Document 2]
[0008] JP-A-1995-192669 [Non-Patent Document 1]
[0009] K. Edinger, V. Yun, J. Meingailis, J. Orloff, and G. Magera,
J. Vac. Sci. Technol. A 15 (No. 6) (1997) 2365 [Non-Patent Document
2]
[0010] J. Morgan, J. Notte, R. Hill, and B. Ward, Microscopy Today
July 14 (2006) 24
[Non-Patent Document 3]
[0011] H.-S. Kuo, I.-S. Hwang, T.-Y. Fu, Y-C. Lin, C.-C. Chang, and
T. T. Tsong, 16th Int. Microscopy Congress (IMC16), Sapporo (2006)
112D
[Non-Patent Document 4]
[0012] H.-S. Kuo, I.-S. Hwang, T.-Y. Fu, J.-Y. Wu, C.-C. Chang, and
T. T. Tsong, Nano Letters 4 (2004) 2379.
SUMMARY OF THE INVENTION
[0013] The inventors of the present application conducted assiduous
investigations on the GFIS and consequently obtained the following
knowledge.
[0014] Ideally, a nano tip is formed at the apex of a W emitter in
the direction of the axial orientation <111>. To check the
ion emission therefrom or align (adjust) this ion emission
direction with the optical axis of the scanning ion microscope, a
field ion microscope (abbreviated to FIM) pattern or corresponding
means is used. In this pattern observation, it is preferable that
the aperture diameter of the extraction electrode be large to such
an extent that an ion beam with a divergence half angle a of about
20 degrees can go through the aperture. However, after the
alignment (adjustment) with the optical axis, the pressure of the
ion material gas (for example, He) which is introduced into the
emitter room is raised to about 10.sup.-2-1 Pa in order to increase
the angular ion current density (emitted ion current per unit solid
angle). This introduced gas is released by differential pumping
through the aperture of the extraction electrode. In order to keep
high the gas molecule density around the tip of the emitter as well
as to reduce the amount of gas evacuated without being ionized, the
aperture diameter is preferred to be small. A first problem found
by the inventors of the present application is to not only allow
the aperture to let widely emitted ions go through but also secure
the differential pumping although the former involves enlarging the
aperture diameter while the latter involves reducing the aperture
diameter. If the nano tip is damaged, the ion emission direction
from the nano tip must be checked again after a nano tip is
reformed.
[0015] To increase the ion current, it is important to increase the
density of gas molecules around the tip.
Since the density n of gas molecules per unit pressure [Pa] is in
inverse proportion to the gas temperature T[K] as given by the
following formula, it is important to cool the gas and the emitter
together.
n[molecules cm.sup.-3Pa.sup.-1]=7.247.times.10.sup.16/T(K) (1)
[0016] The cooling means often includes a physically vibrating
element and therefore may cause the emitter to vibrate. A second
problem found by the inventors of the present application is to
reduce this vibration of the emitter.
[0017] It is an object of the present invention to improve the
stability of the gas field ionization ion source.
[0018] A GFIS of the present invention is characterized in that the
aperture diameter of the extraction electrode can be set to any of
two different values or the distance from the apex of the emitter
to the extraction electrode can be set to any of two different
values.
[0019] A GFIS of the present invention is characterized in that
solid nitrogen is used for cooling.
[0020] According to the present invention, it is possible to not
only let divergently emitted ions go through the aperture of the
extraction electrode but also, in behalf of differential pumping,
reduce the diameter of the aperture. It is also possible to reduce
the physical vibration of the cooling means. Consequently, it is
possible to provide a highly stable GFIS and a scanning charged
particle microscope equipped with such a GFIS.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] Other objects and advantages of the invention will become
apparent from the following description of embodiments with
reference to the accompanying drawings in which:
[0022] FIG. 1 schematically shows the configuration of a gas field
ionization ion source (GFIS);
[0023] FIG. 2A is a diagram for explaining the relation between the
emitter's tip and the hole diameter of the extraction
electrode;
[0024] FIG. 2B is an example of a field ion microscope (abbreviated
to FIM) pattern from the W emitter <111> before a nano tip is
generated;
[0025] FIG. 3 shows an extraction electrode comprising a movable
flat plate electrode which has dimensionally-different apertures
formed in the same plane;
[0026] FIG. 4 shows aperture changing means comprising an
aperture-forming part having an aperture through which ions
extracted by the extraction electrode are passed, and a mounting
part on which the aperture-forming part is mounted;
[0027] FIG. 5 shows an extraction electrode which can be moved in
the direction of the optical axis;
[0028] FIG. 6 shows a gas field ionization ion source which uses
solid nitrogen as the cooling substance;
[0029] FIG. 7 shows a gas field ionization ion source equipped with
a refrigerator by which a cooling substance, obtained by
solidifying a refrigerant gas, is further cooled;
[0030] FIG. 8 is an illustration for explaining an accelerating
lens function between the extraction electrode and the focusing
lens first electrode; and
[0031] FIG. 9 shows curves indicating how the angular magnification
M.sub.ang by the accelerating lens between the extraction electrode
and the focusing lens first electrode is dependent on the
extraction voltage V.sub.ext.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] One aspect of the present invention provides a gas field
ionization ion source having a needle-shaped anode emitter and an
extraction electrode which forms an electric field by which gas
molecules at the apex of the emitter are ionized and extracted,
wherein the diameter of the extraction electrode's aperture for
letting extracted ions pass therethrough can be set to any of at
least two different values.
[0033] Another aspect of the present invention provides a gas field
ionization ion source having a needle-shaped anode emitter and an
extraction electrode which forms an electric field by which gas
molecules at the apex of the emitter are ionized and extracted,
wherein the extraction electrode can be separated into an
aperture-forming part having an aperture for letting extracted ions
pass therethrough, and a base part on which the aperture-forming
part is mounted, wherein the aperture-forming part can be withdrawn
from and set around the optical axis of ions.
The aperture-forming part may be slid with respect to the base
part.
[0034] Another aspect of the present invention provides a gas field
ionization ion source having a needle-shaped anode emitter and an
extraction electrode which forms an electric field by which gas
molecules at the apex of the emitter are ionized and extracted,
wherein the distance from the apex of the emitter to the extraction
electrode can be set to any of at least two different values.
[0035] Another aspect of the present invention provides a gas field
ionization ion source comprising: a needle-shaped anode emitter and
an extraction electrode which forms an electric field by which gas
molecules at the apex of the emitter are ionized and extracted,
wherein the cooling substance for cooling the emitter is a
solid-state substance obtained by solidifying a refrigerant gas
which is in the gaseous state under room temperature and
atmospheric pressure conditions. The refrigerant gas may be
nitrogen.
[0036] Another aspect of the present invention provides a scanning
charged particle microscope comprising: a gas field ionization ion
source as described above; a lens system by which ions from the ion
source are accelerated and focused on a specimen; a limiting
apparatus plate for limiting the ions which are focused on the
specimen; and a charged particle detector to detect charged
particles emitted from the specimen.
[0037] Another aspect of the present invention provides a method
for adjusting the optical axis of a scanning charged particle
microscope as described above, wherein the angular range of emitted
ions allowed to pass through the extraction electrode is set larger
for adjusting the optical axis of the gas field ionization ion
source but smaller than for adjusting the optical axis for using
the scanning charged particle microscope to observe the
specimen.
[0038] Another aspect of the present invention provides a method
for observing a specimen by using a scanning charged particle
microscope as described above, wherein the angular range of emitted
ions allowed to pass through the extraction electrode is set larger
for adjusting the optical axis of the gas field ionization ion
source but smaller than for adjusting the optical axis for using
the scanning charged particle microscope to observe the
specimen.
[0039] Above-mentioned and other novel characteristics and effects
of the present invention will be described below by way of
embodiments with reference to the drawings. Note that the drawings
are used for the purpose of description and do not intend to limit
the scope of the claims. As well, some of the respective
embodiments can be combined as appropriate.
Embodiment 1
[0040] FIG. 1 schematically shows the construction of a scanning
charged particle microscope equipped with a GFIS. The ions 5
emitted from the emitter 1 of the GFIS 4 are focused onto a
specimen 14 by a focusing lens 6 and an objective lens 12. A beam
deflector/aligner 7, a movable beam limiting aperture plate 8, a
blanking electrode 9, a blank beam stop plate 10 and a beam
deflector 11 are disposed between the two lenses. The secondary
electrons 15 emitted from the specimen 14 are detected by a
secondary electron detector 16. A beam controller 17 controls the
GFIS 4, focusing lens 6, objective lens 12, upper beam
deflector/aligner 7, lower beam deflector 11, secondary electron
detector 16 and others. A personal computer 18 controls the beam
controller 17 and processes/stores various data. An image display
unit 19 displays SIM images and control screens of the PC 18.
[0041] FIG. 2A is a diagram for explaining the relation between the
emitter's tip and the hole diameter of the extraction electrode.
FIG. 2B is an example of a field ion microscope (abbreviated to
FIM) pattern from the W emitter <111> before a nano tip is
generated. Major orientations <111> and <211> are
marked on the pattern. A nano tip is formed in the direction of
this <111> orientation. To verify this orientation of
formation, it is preferable that observation of emitted ions be
done so widely with respect to the orientation <111> as to
make observable those emitted in the direction of the orientation
<211> at least. Aperture angle .theta. between orientations
<hk1> and <h' k' 1'> is calculated by using the
following formula. Accordingly, .theta. between orientations
<111> and <211> is calculated to be about 19.5
degrees.
cos .theta. = hh ' + kk ' + ll ' ( h 2 + k 2 + l 2 ) 1 / 2 ( h 2 +
k 2 + l 2 ) 1 / 2 ( 2 ) ##EQU00001##
[0042] If the distance s from the emitter tip to the extraction
electrode is 5 mm, the aperture diameter d.sub.apture required is
2.times.5.times.tan19.5.degree.=3.5 (mm). Since the ion emission
divergence angle is narrowed to 1 degree or smaller after a nano
tip is formed, the aperture diameter d.sub.apture is sufficiently
large if not smaller than 0.2 [mi]. To increase the radiant angular
current density, ion material gas (for example, He) is introduced
into the nano tip room to a degree of vacuum of approximately
10.sup.-2-10 Pa. Behind the extraction electrode, the ambience
surrounding the focusing lens, objective lens and specimen is
highly evacuated. In the aspect of differential pumping,
d.sub.apture=0.2 [mm] is valid.
[0043] The distance s is set by considering not only this ion
emission divergence angle but also that excessively shortening the
distance causes electric discharge between the emitter and the
extraction electrode while excessively lengthening the distance
causes collision between emitted ions and introduced He gas atoms
(or molecules). This collision deteriorates the beam focusing
characteristic of the scanning charged particle microscope since
the traveling directions of emitted ions are bent and therefore the
virtual source size of the ion source enlarges substantially. By
using the gas molecule density n and diameter a, the mean free path
A. of the emitted ions can be calculated from the following
formula.
.lamda. = 2 1 n .pi..sigma. 2 ( 3 ) ##EQU00002##
[0044] For He molecules (a-0.22 nm), the above formula is rewritten
as below by denoting the gas temperature as T[K] and pressure as p
[Pa].
.LAMBDA.[cm]=6.4E-3(T/p) (4)
[0045] For example, if p=5Pa, X is 3.5 and 1.0 [mm] at room
temperature (T=273K) and liquid nitrogen temperature (T=77K),
respectively.
[0046] In the present embodiment, means to change the aperture
diameter d.sub.apture of the extraction electrode 3 is employed.
Specifically, a fixed electrode 3a having a large aperture (for
example 6 mm in diameter) is combined with a movable flat plate
electrode 3b having two dimensionally-different apertures
(d.sub.apture=0.2 and 3.5 [mm]) formed in the same plane (Refer to
FIG. 3). The center of the fixed electrode's large aperture is
aligned with the optical axis 20 of the scanning charged particle
microscope. Through moving operation from the atmospheric side, it
is possible to move the movable flat plate electrode 3b while
keeping the movable flat plate electrode 3b perpendicular to the
optical axis. Thus, either of its large and small apertures can
selectively be aligned with the optical axis. Although two
dimensionally different apertures are available in the present
embodiment, three or more holes may be prepared. Increasing the
number of such dimensionally-different apertures directly widens
the assortment of adjustment/choice for the differential pumping
described later. Since high voltage is applied to the extraction
electrode 3 when the GFIS is mounted to the scanning charged
particle microscope, the movable flat plate electrode 3b is
insulated from the microscope column (not shown in the figure) at
the ground potential.
Embodiment 2
[0047] The present embodiment described below is a scanning charged
particle microscope provided with changing means to change the
aperture diameter d.sub.apture of the extraction electrode 3 which
differs from the changing means employed in embodiment 1. The
following description is focused on what are unique to the present
embodiment.
[0048] The changing means of the present embodiment is structurally
similar to the variable aperture employed in cameras and others.
Plural diaphragm blades are combined so as to have a circular
aperture which can coaxially be varied in diameter by changing the
amount of overlap between diaphragm blades. By employing such means
to change the aperture diameter of the extraction electrode, it is
possible to not only let widely emitted ions go through but also,
in behalf of differential pumping, reduce the diameter of the
aperture.
Embodiment 3
[0049] The present embodiment is a scanning charged particle
microscope provided with changing means to change the aperture
diameter d.sub.aperture of the extraction electrode 3 which differs
from the changing means employed in either embodiment 1 or 2. The
following description is focused on what are unique to the present
embodiment.
[0050] As shown in FIG. 4, the changing means of the present
embodiment can be separated into an aperture-forming part 3d having
an aperture through which ions extracted by the extraction
electrode are passed, and a mounting part 3c on which the
aperture-forming part 3d is mounted. The aperture-forming part 3d
can be moved to and withdrawn from the optical axis 20. The
aperture-forming part 3 is located as indicated with reference
numeral 3d' if the aperture-forming part 3 is withdrawn from the
optical axis 20 by sliding it on the mounting part 3c.
Embodiment 4
[0051] Like embodiments 1 through 3, the present embodiment intends
to solve the problem of not only letting widely emitted ions go
through but also, in behalf of differential pumping, reducing the
diameter of the aperture. However, a different approach is taken by
the present invention to solve the problem. Specifically, the
extraction electrode 3 (d.sub.apture=1 [MM]) is provided with means
to move it in the axial direction. The following description is
mainly focused on what are unique to the present embodiment.
[0052] FIG. 5 schematically shows the extraction electrode which
can be moved in the direction of the optical axis.
[0053] Reference numeral 3' indicates the same extraction electrode
after it is moved. Distance s from the emitter tip to the aperture
of the extraction electrode can be set to any of two values 1 and 5
[mm]. s=1 mm corresponds to an ion emission divergence half angle a
of about 27 degrees while s=5 mm to about 6 degrees. By thus moving
the extraction electrode in the axial direction, it is possible to
not only let widely emitted ions go through but also, in behalf of
differential pumping, reduce the diameter of the aperture.
[0054] If the aperture diameter d.sub.apture of the extraction
electrode 3 is 1 [mm] in combination with s=1 mm, it is possible to
it is possible to not only let widely emitted ions go through but
also, in behalf of differential pumping, reduce the diameter of the
aperture. However, discharge is likely to occur between the emitter
tip and the extraction electrode if the pressure p of the ion
material gas is raised in order to raise the brightness. S=5 mm is
for preventing this discharge. However, if s is excessively large,
ions emitted from the emitter may collide with gas molecules, which
causes undesirable results such as deflected trajectories and
reduced kinetic energies of ions. In addition, this change of s is
accompanied by a change in the strength of the electric field
formed at the tip of the emitter although the emitter potential is
fixed.
Consequently, the ion current changes largely since the ionization
efficiency changes. Therefore, to reduce the change of the ion
current, there is provided a select mode for enabling/disabling
adjustment of the extraction voltage.
[0055] Although the present embodiment changes the emitter
tip-to-electrode distance s discontinuously to one of two values,
namely 1 and 5 [mm], continuous change is preferable since
continuous adjustment is possible. In addition, although the
present embodiment changes the distance s to one of the two values
by moving the extraction electrode in the axial direction,
substantially the same effect can be obtained by moving the emitter
in the axial direction with the extraction electrode fixed.
Embodiment 5
[0056] To attain a high ion current, it is important to cool the
ion material, i.e., introduced gas as well as the ion emitter. In
the case of He gas, cooling down to about 10K is desirable.
However, such a cooling device usually generates physical vibration
and propagate it to the emitter. Vibration of the emitter causes
the scanning charged particle microscope to vibrate the beam
irradiation spot on the specimen, resulting in a lowered resolution
of the microscope. It is difficult to stop the propagation of
physical vibration from the cooling device to the emitter.
Accordingly, the present embodiment employs solid nitrogen
(solidification point in vacuum: about 51K) as the cooling
substance. The following description is focused on what are unique
to the present embodiment.
[0057] FIG. 6 schematically shows the construction of the ion
source. Into the vicinity of the emitter 1, ion material gas,
namely He gas 32 is introduced via a thin gas supply pipe 33. Solid
nitrogen 34 is used as the cooling substance. Liquid nitrogen 30,
firstly introduced into the refrigerant room 36 from a supply pipe
31, becomes solid nitrogen 34 since the vaporized nitrogen is
evacuated through the exhaust pipe 35. The solid nitrogen in the
evacuated environment cools the emitter and introduced gas as the
solid nitrogen absorbs heat from them and sublimes. This is quite
effective in lightening the vibration of the emitter tip since
unlike liquid nitrogen, sublimation does not generate bubbles which
cause physical vibration. To sufficiently cool the emitter, it is
preferable to cool the emitter voltage application wire 37, the
control electrode voltage application wire 38 and the extraction
electrode 3. In addition, a low heat conduction material is used
for junction between the cooled section and the room temperature
section in behalf of radiation shield against inrushing heat from
the room temperature section into the cooled section by thermal
radiation. This cooling means is much more compact and inexpensive
as compared with He cooling means aimed at cooling down to about
10K.
[0058] The cooling substance of the present embodiment is
characterized in that it is obtained by solidifying a refrigerant
gas which is in a gaseous state under room temperature and
atmospheric pressure conditions. Accordingly, the refrigerant gas
may be hydrogen (melting point 14K and boiling point 20K at
atmospheric pressure), neon (melting point: 24K, boiling point:
27K), oxygen (melting point: 54K, boiling point 90K), argon
(melting point: 84K, boiling point: 87K), methane (melting point:
90K, boiling point: 111K) or the like instead of nitrogen (melting
point: 51K, boiling point 77K). In terms of cost and safety,
nitrogen is superior.
Embodiment 6
[0059] Although embodiment 5 uses a cooling substance obtained by
converting a refrigerant gas into a solid state, such a solid
cooling substance is further cooled in the present embodiment. The
following description are focused on what are unique to the present
embodiment.
[0060] FIG. 7 schematically shows a gas field ionization ion source
equipped with a refrigerator by which a solidified cooling
substance is further cooled. The refrigerant gas in this example is
nitrogen. Firstly, liquid nitrogen 30 is introduced into the
cooling substance room 36 from the supply pipe 31. The cooling head
51 of the He refrigerator 50 is arranged within the .cooling
substance room 36, and cooling metal rods 52 connected thereto are
extended into the liquid nitrogen. The liquid nitrogen is converted
to solid nitrogen 30 since the vaporized nitrogen is evacuated
through the exhaust pipe 35. Then, the solid nitrogen is cooled
further to a temperature below the melting point by the
refrigerator when turned on.
[0061] For observation with the ion microscope, the refrigeration
is turned on. As compared with dependence on the solid nitrogen
alone, this lowers the emitter temperature further by about 20K and
consequently raises the brightness of the ion source. The
refrigerator may also be turned off to suppress the physical
vibration due to the refrigerator when observation is performed
with the ion microscope.
Embodiment 7
[0062] The present embodiment is described below with reference to
FIG. 8 and FIG. 9. In the present embodiment, the direction of ion
emission from the nano tip of the emitter is checked and the ion
emission direction is aligned (adjusted) with the optical axis of
the scanning ion microscope while observing a quasi-FIM
pattern.
[0063] Ions 5 emitted divergently from the emitter 1 pass by the
focusing lens (whose lens function is turned off by setting the
lens potential V.sub.L to the ground potential) and arrive at the
movable beam limiting aperture plate 8. The ion beam which has
arrived thereat partly passes through the aperture of the movable
beam limiting aperture plate 8. Irradiated with the ions which have
passed, the specimen 8 emits secondary electrons 15. The secondary
electrons 15 are detected by the secondary electron detector 16.
Above the movable beam limiting aperture plate 16, the beam
deflector/aligner 7 deflects the beam according to a scan signal. A
signal. synchronized with this scan signal and the intensity
detected by the secondary electron detector 16 are used
respectively as the XY signal and Z signal (brightness) to generate
a SIM image. This SIM image is monitored on the image display unit
19. The movable beam limiting aperture plate 8 can be moved in a
plane perpendicular to the optical axis, allowing fine optical axis
or XY adjustment. In addition, the aperture diameter thereof can be
selected from various values in a wide range. In the present
embodiment, the lens function of the objective lens 12 is adjusted
so that the deflection fulcrum of the beam deflector/aligner 7 is
projected onto the specimen 14. As a result of this adjustment,
although beam scanning by the beam deflector/aligner 7 is
performed, the specimen is not scanned by a beam. Rather, the SIM
image on the monitor screen shows the angular intensity
distribution of emitted ions wherein the X and Y axes represent the
emission angles measured toward the X and Y directions
respectively. While a FIM image has such a resolution that the ion
emission region of the emitter is projected at an atomic level,
this SIM image corresponds to an abridged and blurred FIM image
which covers an ion radiant solid angle associated with the
aperture of the movable beam limiting aperture plate 44. When the
scan function of the beam deflector/aligner 7 is turned off, fine
XY adjustment and aligner adjustment of the beam deflector/aligner
7 are performed such that the ion emission direction <111>
for the quasi-FIM image passes through the center of objective lens
12 and the aperture center of the movable beam limiting aperture
plate 8.
[0064] In FIG. 8, the focusing lens 8 is an electrostatic lens
constituted of three electrodes (6a, 6b and 6c). The two outermost
electrodes are at the ground potential. Between the extraction
electrode 3 and the focusing lens first electrode 6a, there is an
ion accelerating lens function. By using .alpha.o to denote the
angle of the ions input to this lens and .alpha.i to denote the
angle of the ions output therefrom, its angular magnification
M.sub.ang is defined by the following formula.
M.sub.ang=(.alpha./.alpha.o (5)
[0065] If no accelerating lens function is given, that is, the
acceleration voltage (V.sub.acc) is set equal to the extraction
voltage (V.sub.ext), M.sub.ang becomes equal to 1. FIG. 9 shows an
example of curves indicating how M.sub.ang is dependent upon
V.sub.ext if the ion acceleration voltage V.sub.acc is fixed to 25
kV and the distance Z.sub.acc between the extraction electrode 3
and the focusing lens first electrode 6a is fixed to 20 mm. These
curves are plotted for s=3, 5 and 7 mm respectively. Positive and
negative M.sub.ang values indicate respectively that the output
ions are diverged and converged. If the M.sub.ang is zero, the
output ions are parallel to the optical axis. As understood, the
beam diameter at the movable beam limiting aperture plate 8 varies
depending on M.sub.ang even if the focusing lens is off since the
divergence angle of the ions 5 emitted from the emitter is
multiplied by a factor of M.sub.ang due to the acceleration lens
function. That is, the optimum aperture diameter of the movable
beam limiting aperture plate 8 varies depending on these values. To
adjust the optical axis when the GFIS is mounted to the scanning
charged particle microscope or to correct/observe the formation or
regeneration of a nano tip on the emitter end, V,acc is set lower.
For regular operation of the scanning charged particle microscope
after that, Vacc is raised to a certain level.
[0066] When adjusting the optical axis of the GFIS in the scanning
charged particle microscope (for example, after the emitter tip is
repaired), its field emission pattern is monitored by allowing
divergently emitted ions to pass the extraction electrode. When
using the scanning charged particle microscope to observe a
specimen, less divergently emitted ions are allowed to pass through
the extraction electrode. By this setting, it is possible to
smoothly and efficiently perform high accuracy optical axis
adjustment and specimen observation.
[0067] While the invention has been described in its preferred
embodiments, it is to be understood that the words which have been
used are words of description rather than limitation and that
changes within the purview of the appended claims may be made
without departing from the true scope and spirit of the invention
in its broader aspects.
* * * * *